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[TowerTalk] 75 ohm feedline

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Subject: [TowerTalk] 75 ohm feedline
From: (L. B. Cebik)
Date: Mon, 7 Jul 1997 07:03:48 -0400 (EDT)
> As Pete stated....the loss in having a 75ohm feedline going to a 50ohm
> antenna is minimal.  It would be interesting to see if the transformers
> in-line would cause more harm (be it a small amount) than good.

Here are a couple of notes that may be useful in deciding whether or not
to use transformers with a run of 75 ohm cable with a 50-ohm antenna and a
transmitter/amp designed for 50-ohm systems.

Assuming the antenna present 50 ohms resistive (no reactance), the SWR to
75 ohm cable is 1.5:1.  Along a lossless line (where peaks are the
highest), the highest impedance occurs at 90-degree points, where it is
112.5 ohms (resistive).  Maximum reactance occurs at 55 degrees (31.2
ohms) and at 125 degrees (-31.2 ohms).  These values were derived from a
little utility I use that calculates at 5 degree increments, so the
reactance peaks might be slightly higher at "midway" points between
reading points.  If the cable has a rating of more than 1000 volts and can
handle 6 amps or so, then nothing destructive occurs.  Losses will, of
course, vary with the type of cable selected.  They are with hard line
very low, with <.1 dB loss added for running it with the 1.5:1 SWR.

The above scenario presents nothing destructive, but it may present some
equipment with values that force excessive retuning due to sharpness of
settings under some line length conditions.  The recommendations for line
lengths of 1/2 wl or multiples tend to yield a return to the 50 ohms
resistive load at the transmitting end and the flattest setrtings (or most
broadbanded) for least retuning with small changes of frequency.  The idea
of terminating a hardline and using a cuttable length of RG-11 for the
happiest overall length is wise, because it permits finding the 50-ohm
"flat" spot even if there is some reactance at the antenna junction of the

Well designed transmission line transformers with a 1.4:1 or 1.5:1 ratio
are exceptionally efficient so long as the reactive component of the
antenna impedance is low.  A 2:1 SWR range at the antenna with a centeer
50-ohm point is a good rule of thumb for estimating whether reactance is
low enough for >99% efficiency of the transformers.  This note of course
cannot address the best way in all weather conditions of maintaining the
transformer in good operating condition, but assuming this problem is
overcome, losses less than 0.1 dB for well-designed transmission line
transformer pairs are reasonable.

It would appear that we have a classic "6 of one, half-dozen of the other"
scenario:  cut your line with precision to keep the equipment adjustments
flat and minimal or use transformers with attention to proper wx treatment
for your area and climate for the same purpose, with comparable
losses--which are negligible in either case relative to all the other loss
sources that might enter such systems.  No wonder folks have claimed
success for both ways of doing things.



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